Reflective and slanted array channelized sensor arrays

ABSTRACT

Reflective and slanted array channelized sensor arrays having a broadband source providing acoustic energy to a reflective or slanted array that reflects a portion of the incident signal to one or more sensing films wherein the response is measured.

FIELD OF THE INVENTION

The invention relates to sensors, and more particularly, to dispersiveacoustic sensing technology.

BACKGROUND OF THE INVENTION

A sensor is essentially a device that responds to a physical, chemical,biological, or electrical stimulus by generating an electrical outputsignal that is related to the input electrical signal and a condition tobe measured. There have been significant advances in the sensingindustry based on requirements and applications in diverse fields suchas biochemical sensing for security to curing of concrete. For example,the airport screening measures employ sensors for explosives,radioactive and/or biological dangers. Unfortunately, the presentsensing technology lacks the sensitivity and ability to accuratelydetect a broad range of stimulus. This is particularly true with respectto employing the electronic sensors in environments which are unable toisolate the sensitive electronic components from the surroundingconditions.

An acoustic wave sensor responds to some input stimulus by thetransconduction of the input stimulus to a perturbation of theelectronic properties of the acoustic wave device. An output signal isproduced by the acoustic wave sensing that represents a function of theinput signal, the sensor element's response and the environmental inputstimulus. In most cases the acoustic wave device is a passive circuitelement, having an output that is at the same frequency as the inputsignal. Only the transfer efficiency and phase at a given excitationfrequency typically change; however there are circuits that adjust thefrequency of a signal to follow a phase or amplitude condition. It isoften said that the frequency changes in such devices when, in fact,only velocity and length change at a physical level.

The acoustic wave sensor typically has a sensing area, such as a sensingfilm, wherein the device is sensitive to mechanical and electricalperturbations such as mass loading, viscoelastic property variations,electroacoustic interactions, flexure-frequency effects, andforce-frequency effects. The sensor responds to the input stimulus witha corresponding change in the resonant frequency of the acoustic wavedevice or the phase shift and/or amplitude of the device at a specifiedfrequency such that the change in frequency can be used to indicateproperties of the stimulus. The use of acoustic wave sensors has led tomany new applications and uses including in-field applications.

Current acoustic wave gas sensors are typically based on mass loading ofa sensing film upon exposure to a target analyte. Mass loading refers tomeasuring changes of the vibrating member due to an increase of the masscaused by an adsorption of some gas. A mass loaded resonator haselectrodes that convert energy between electrical and acoustic energy,wherein the device vibrates at a frequency determined by the inputelectrical energy signal. The degree and ease with which vibrationoccurs is determined by the proximity of the frequency of excitation tothe device's resonant frequency or frequencies. In some cases theexternal circuit is devised so as to maintain the excitation at theresonant frequency as in an oscillator or phase locked loop. In others,a property of the signal is modified in a manner determined by theexcitation frequency's proximity to said resonances. As the gasmolecules are adsorbed by the sensing film, the added mass of the gasmolecules causes a change in the propagation or resonance of theacoustic wave device. For such a device the resulting change is afrequency decrease in an oscillator or a phase shift increase at a fixedexcitation frequency.

Some examples of the current type of sensors include capacitance-basedsensors. These devices tend to use thick film polymers to form a sensorarray. Another type of sensor is a SiC resonator which typically uses apre-concentrator to increase sensitivity. The SiC resonator typicallyuses thick film polymers to construct a sensor array, such as 2-5microns and uses mass loading for detection via induced shifts in theresonant frequency of the SiC plate.

Surface generated acoustic wave (SGAW) devices and Bulk Acoustic Wave(BAW) devices are used in many sensing applications, wherein a change infrequency of the sensor is related to the amount of mass that getsadsorbed onto or absorbed into the sensing film.

The limitation of current technology is that the sensing films in usehave limited selectivity and customers demand proper identification ofthe analyte. Customers also seek multifunctional sensors that arecapable of identifying and measuring multiple analytes in a mixture.This has led to the use of sensor arrays containing several discretesensor elements, typically limited in the published art to four or eightsensors by manufacturing and design constraints.

Due to the inherent manufacturing variations and signal cross-talkissues, these arrays consist of sensors at different frequencies withunused guard bands between them. This results in unnecessarily tightmanufacturing tolerances and limited array sizes in the crampedelectromagnetic spectrum for wireless sensors and a generally difficultburden of multiplicity of design and complex spurious signalinteractions for wired sensors. One general feature of the presentinvention is to incorporate an array of distinct sensing mechanisms intoa single sensor.

SUMMARY OF THE INVENTION

The present invention according to one embodiment relates to sensingtechnology based on responses to sensing materials and structures.

One embodiment is a sensing apparatus comprising an acoustic energysource providing an input acoustic energy to a first reflector array,wherein the first reflector array has a plurality of individualreflectors reflecting a portion of the input acoustic energy, andwherein the first reflector array provides a reflected signal withdiffering frequencies. The frequencies can be continuously varying orseparated into frequency bands depending upon the arrangement of theindividual reflectors. There is at least one sensing region proximatethe reflector array, wherein at least some of the reflected signalimpinges upon the sensing region providing an altered reflected signal.The sensing region can be a continuous sensing material with the same orvarying sensing particulars. Alternatively, the sensing material can bea plurality of sensing areas. A transducer can be used to receive atleast some of the altered reflected signal.

The individual reflectors, in another embodiment, are selected from thegroup consisting of continuously varying individual reflectors andsections of similar individual reflectors.

An additional feature includes a second reflector array proximate thesensing region with the sensing region disposed between the firstreflector array and the second reflector array, wherein the secondreflector reflects at least some of the altered reflected signal.

The transducer receiving the altered reflected signal can be selectedfrom the group consisting of a common transducer which also provides theinput energy source and a separate output transducer.

A further embodiment includes at least one of an in-line gratingreflecting at least some of the input acoustic energy, an in-linegrating reflecting at least some of the reflected signal, an in-linegrating reflecting at least some of the altered reflected signal, and atleast one end reflector reflecting at least some of the alteredreflected signal.

A further aspect includes having an absorber material proximate at leasta portion of a periphery of the apparatus.

In addition, a reference transducer receiving at least some of the inputacoustic energy is a further feature.

At least one microelectromechanical system (MEMS) device proximate thesensing region can be used to provide gating for the reflected signal ifthe MEMS is before the sensing region or for the altered reflectedsignal if located after the sensing region.

A further embodiment includes a medium on at least a portion of thesensing region, wherein the medium is selected from at least one of thegroup consisting of a blocking medium and a phase shift medium. Theblocking medium can provide better isolation and/or coding. The phaseshift can be used to discriminate and otherwise provide for coding.

A matching medium can be disposed on at least a portion of the firstreflector array to provide a better transition between the reflectorsegments.

The apparatus according to one embodiment includes having the apparatus,with its various components, disposed upon a shear horizontal (SH) RAC(SH-RAC) substrate.

In yet another feature, the sensing region is selected from at least oneof the group consisting of polymer films, metal films, metal oxidefilms, enzymes, antibodies, DNA, and thin membranes.

One embodiment is a sensor system for detecting a target analyte,comprising providing an initial acoustic energy signal, reflectingportions of the acoustic energy signal by a reflector array to a sensingregion, producing at least one altered acoustic energy signal, whereinthe reflector array has a plurality of individual reflectors, andwherein the reflector array reflects more than one frequency. The systemprovides for receiving the altered acoustic energy signal, convertingthe altered acoustic energy signal into an altered energy signal andmeasuring a change between the initial energy signal and the alteredenergy signal for detecting the target analyte.

An additional aspect includes reflecting the altered acoustic energysignal by an additional reflector array to an output transducer.

The acoustic energy source in one embodiment is a common transducer anda further step is reflecting a portion of the altered acoustic energysignal back to the common transducer.

An additional feature is coding by at least one of the group consistingof disposing a blocking medium on portions of the sensing region anddisposing a phase shift medium on portions of the sensing region.

One further embodiment is a slanted array sensor comprising a slantedsource transducer converting a broadband electrical energy signal into aplurality of channels of narrow-band acoustic energy signals. There isat least one sensing region proximate the slanted source transducer,wherein at least one of the narrow-band acoustic energy signals impingesupon the sensing region and produces at least one altered acousticenergy signal. A slanted receiver transducer proximate the sensingregion is provided, wherein the slanted receiver transducer receives atleast one of the altered acoustic energy signals.

In addition, the slanted source transducer can be a slanted fingerinterdigital transducer and the slanted receiver transducer can be aslanted finger interdigital transducer.

Another feature includes a phase plate disposed between the slantedsource transducer and the slanted receive transducer.

The features and advantages described herein are not all-inclusive and,in particular, many additional features and advantages will be apparentto one of ordinary skill in the art in view of the drawings,specification, and claims. Moreover, it should be noted that thelanguage used in the specification has been principally selected forreadability and instructional purposes, and not to limit the scope ofthe inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

A full understanding of the invention can be gained from the followingdescription when read with the accompanying drawings:

FIG. 1 a is a reflective array compressor structure showing thedifferent frequencies reflected onto sensing materials according to oneembodiment.

FIG. 1 b is a reflective array channelized (RAC) sensor with multiplediscrete oblique reflector segments according to another embodiment.

FIG. 1 c is a reflective array channelized (RAC) sensor with multipleoblique reflector segments at the different frequencies reflected ontosensing films including a guard band medium and a phase shifting mediumaccording to another embodiment.

FIG. 2 is a portion of a reflective array channelized (RAC) sensorshowing a common transducer, oblique gratings, and in-line gratingsaccording to one embodiment.

FIG. 3 is a graph showing a signal response of the extracted signals atthe in-line and oblique reflectors as well as the transmitted signalaccording to an embodiment of the invention.

FIG. 4 is a reflective array channelized (RAC) sensor with a referenceoutput according to one embodiment. A blocking medium implements afrequency guard band or encoding scheme.

FIG. 5 is a reflective array channelized (RAC) sensor with multiplesegments with the output transducer opposing the input transduceraccording to one embodiment.

FIG. 6 is a reflective array channelized (RAC) sensor with a commontransducer including in-line reflectors and end reflectors to implementa one-port echo-based sensor according to one embodiment.

FIG. 7 is a reflective array channelized (RAC) sensor showing adifferent angular orientation according to one embodiment.

FIG. 8 is a reflective array channelized (RAC) sensor with a commontransducer including a common chirped input reflector according to oneembodiment. The figure also illustrates several acoustically coupledresonators coupled to the input reflector.

FIG. 9 is a reflective array compressor sensor with a common transducerincluding a common input reflector and a common slanted end reflectoraccording to one embodiment, implementing a one-port reflective arraycompressor.

FIG. 10 is a slanted array channelized (SAC) sensor showing the discretefrequency segments according to one embodiment.

FIG. 11 is a slanted array channelized (SAC) sensor showing the discretefrequency segments including a phase plate according to one embodiment.

FIG. 12 shows a slanted finger interdigital transducer channelizedsensor according to one embodiment.

FIG. 13 shows one embodiment of a block diagrammatic perspective of thesensor system disposed on a substrate.

DETAILED DESCRIPTION

A broad objective of the present invention is to improve current sensortechnologies. Some of the improvements are based on the use ofreflective array sensors and slanted arrays sensors.

In one embodiment by way of illustration, the sensing region experiencesan increase in mass or an increase or decrease in elastic propertiesthat causes a corresponding change in velocity in the propagation pathof the sensor, said velocity change altering a phase shift or resonantfrequency. The state of the art suggests a plurality of means by whichan SGAW or BAW sensor may interact with the environment directly ormediated by a sensing film. The present invention discloses systems,devices, methods and apparatus for employing known sensing techniquesand for brevity some of the methods themselves are not detailed herein.For illustrative purposes the discussions in the embodiment focus on anidealized polymer having a selective absorption of a target gas andpresenting a pure mass loading sensing mechanism. Other effectsintroduce analogous sensor responses, although at a much less intuitivelevel of presentation. This single example will be understood torepresent mass loading, viscoelastic changes, intrinsic stress effects,film induced flexure of the substrate, local temperature changes,electrostriction, magnetostriction, conductive or dielectric loading,strain (due to torque or pressure as well as the relief of film inducedstress) and the myriad other possible physical, chemical and biochemicalinteractions.

As used herein, the term acoustic wave device shall be designated in abroad sense to include any device that operates as a resonator such asSAW, BAW, and TSM resonators, and is not limited to a particularmaterial, shape or cut. One embodiment of the invention herein isdisclosed with adequate enabling steps specifically for surfacegenerated acoustic wave (SGAW), a term defined by John Vetelino (see forexample, Theory, design and operation of surface generated acoustic wavesensors, Vetelino et al, Microwave Symposium Digest, 1994., IEEE MTT-SInternational, 23-27 May 1994 Page(s):505-508 vol. 1) to mean anyacoustic wave that is generated at, detected at, and interacts with thesurface of the piezoelectric. It includes SAW (SH and Rayleigh), leakySAW, Love, Lamb, acoustic plate mode, shallow bulk acoustic wave,surface skimming bulk wave, and the like. Nonetheless, the use offerroelectric domain reversals allows similar reflective gratings to beimplemented for dispersive oblique reflections in a BAW device and thissort of extension is contemplated.

While the descriptions make reference to a piezoelectric SGAW using aninterdigitated transducer, other sources/detectors are within the scopeof the invention, such as using a wedge transducer to convert thethickness extensional wave of an external BAW transducer into theRayleigh mode of a SAW and to subsequently detect said acoustic wave asan electrical signal. The methods and structures contemplated hereintherefore extend directly to macroscopic mechanical devices usingstainless steel or other non-piezoelectric media.

As used herein, reflector refers to any element that reflects energy,and may refer to individual reflectors. These individual reflectors canbe arranged in so-called chirped arrays to be continuously varying withrespect to reflected frequency or arranged into groups of predominatelylike reflectors wherein each group reflects a different frequency.Similarly, discussions of gratings are not limited in scope to metalgratings or etched grooves. According to one embodiment, gratings referto any periodic plurality or pseudoperiodic chirped plurality ofreflectors. Any quasiperiodic perturbation to the impedance of theacoustic wave will function as a grating and could be incurred throughelectrical effects such as periodic doping of a semiconductor orferroelectric domain reversals. These principles apply wherever areflector or grating is described herein.

For reference purposes, numerous examples and applications of SAWdevices are described in the text by Colin Campbell in “Surface AcousticWave Devices and Their Signal Processing Applications”, Academic Press1989.

The sensing according to one embodiment described herein is based on theability to physically spread a broadband signal into a geometricallydisperse spectrum of frequencies, much like a prism bending white lightinto a rainbow. An example of this effect is the oblique angledreflector grating of the well-known reflective array compressor. A long,locally periodic, reflective grating is designed using slantedelectrodes with a spatially varying period, called a chirped period. Themanufacture of the reflective array compressor typically employsthousands of etched grooves to form the reflector elements. Anotherknown manufacturing technique replaces the grooves with an array of dotsto form a reflective dot array (RDA). The weighting via the grooves wasimplemented by controlling the depth of the grooves of the surface ofthe substrate, however the RDA weighting depends upon the number of dotsand distribution of the dots. The RDA has the advantage of presenting amore continuous wave impedance as reflective angles are changed fromoblique to in-line and as reflectivity is modulated in amplitude.

A reflective array compressor uses a broadband input electrical energysignal source and applies the signal to a transducer to excite theacoustic waves which impinge upon an array of reflective elements havingvaried locally-periodic spacing. In a monolithic structure this inputsource is typically an interdigital transducer or an analog variationthereof. In other implementations, such as a sensor using a stainlesssteel rod or bar, the transducer is the well known wedge transducer.

Different frequency components are reflected at predetermined regions bythe local periodicity of the oblique angled reflective gratings,experiencing different propagation lengths, delay times, and phaseshifts.

In a two port device, a corresponding oblique angled grating reassemblesthe frequency spectral components back into a second signal path whichis directed to another transducer. In a one port device, a singletransducer is used to transmit the acoustic waves and to measure theacoustic waves that are reflected back to the transducer by additionalreflectors at the ends of the transverse paths.

In the signal processing device it is typically desired that the phasebe quadratic with frequency (delay time vary linearly with frequency).The structure of FIG. 1 a provides a mechanism with continuously varyingpath length as a function of frequency. Deviations from the ideal phaseversus frequency response can be corrected using a phase plate (notshown) which slows the local wave by a phase shift proportional to itswidth. By tailoring this local width to correct the phase aberrations ofthe local frequency components it is possible to compensate the impulseresponse of the RAC.

Referring again to FIG. 1 a, one embodiment based on this signalprocessing structure with continuous frequency reflectors illustrates abroadband source 101 generating acoustic energy to an input obliqueangled reflector array 102. The input array 102 comprises a plurality ofreflectors of variable pitch (local center to center spacing) andangular orientation to the incident input beam from the source 101.

The individual angled grooves or reflectors 107 individually divert fromthe broadband beam a portion of the signal from the broadband source101. The term reflectors as used herein refer to any element reflectingenergy. The signals diverted by the input angled reflector array 102 areincident upon some section of the sensing material 104 and furtherincident upon the output angled reflector array 106 that directs theincident beam to the output transducer 108. Subsequent processing (notshown) extracts the required information to determine the attributes ofthe sensing.

The sensing material 104 shown in this embodiment is a single continuousmaterial, such as a tape, or the substrate itself, with a plurality ofsensing areas 103 disposed thereon. As detailed herein, each of thesesensing areas 103 can be for different target sensing. According to oneembodiment, such a sensing material 104 can have a plurality of sensingfilms of various types and sizes. Sensing materials such as films, metaloxides, and even thin membrane to work with localized stress are knownto those skilled in the art. The sensing region 104 is essentially anyregion in which the acoustic energy of a channel interacts with aparameter to be measured.

By placing sensor coating(s) or films 104 proximate the reflectivegratings 102, 106, it is possible to measure the frequency-dependenteffects of the sensing film. Such an implementation allows for analyteinteraction for improved selectivity and/or measurement of amultiplicity of analytes using a linear array of sensing film “pixels”,each sensing pixel affecting a predetermined frequency channel. Thisallows a single acoustic wave sensor to measure tens or even hundreds ofdifferent targets. Recent advances in the application of small dots ofsensing material enable large arrays; however until now no suitabledevice structure allows such a sensor element capable of employing thelarge number of possible sensing pixels.

The reflectors or grooves 107 are disposed on or about a surface of asubstrate with a geometry and groove period 105 defining an incidentacoustic beam along which is applied the broadband signal source 101.The spectrum exiting the input reflective grating 102 has differentfrequencies (F₁-F_(N)) along different portions of the reflector 102,allowing the various frequencies to strike different portions of asensor material 104. In this embodiment the high frequency signals areextracted first as noted by the fine pitch or smaller groove period 105.

The output reflector array 106 may be used in which the associatedportion of the frequency spectrum is mutually reflected back into acollinear broadband signal that is coupled to an output transducer 108.In the case of 90° reflection this may be accomplished with the outputarray 106 being approximately identical and parallel to the inputreflector array 102 thereby providing a dispersive acoustic device.Dispersive devices are typically implemented by reflecting the signalsin a “U” path while nondispersive devices are obtained by reflecting thesignal in a dog-leg path or a Z-path.

The specific angle of the reflector elements 107 of the input array 102is based on the desired exit angle of the acoustic beam to the sensingfilm 104 and the anisotropy of the substrate of the device. In typicalsignal processing, the beam is diverted by the input slanted reflectorarray 102 at right angles however there are other applications that canreflect the beam at other angles. For example, three-fold symmetry ofthe crystal may indicate that reflections of 60° are preferable and theuse of certain highly temperature compensated off-axis cuts may requireother angles. In the reflective array compressor, from which thisembodiment is derived, it is typical to employ right angle reflection,however symmetry considerations may dictate other options, and otherangles are within the scope of the invention. Likewise, the desired exitangle from the output reflector array 106 to the output transducer 108typically matches the angled reflection of the input reflector 102however other output reflection angles are within the scope of theinvention. It should be noted that in cases other than 90° reflection,either complementary reflection angles, φ and 180-φ, must be employed orthe path length through the various sensor films will differ. Both casesrepresent embodiments within the scope of the invention.

The sections of grooved elements 107 or fingers permitting the angledreflection are arranged such that they act as efficient acoustic wavereflectors for different signal frequencies. The effective number offingers that interact with the wave at frequency, f_(i), and is definedas N_(c). For a typical reflective array compressor structure, this canbe approximated by:

Nc÷Np≅fi÷fo√{square root over (TB)}

Where Np=total number of electrode pairs; fo=center frequency ofinterest; fi=excitation frequency; TB=time-bandwidth product of thedispersion (T) and the bandwidth (B). N_(c) must exceed the minimumnumber of fingers required to reflect the signal of interest, which canbe approximately stated as

kN_(c)˜1,

where k is the reflective coupling per element.

The angle for the fingers 107 of the input reflector 102 to accomplishthe desired slanted reflection from the broadband source 101 that isdirected to the sensing material 104 and subsequently to a correspondingmatching group of fingers on the output reflector 106 would normallyhave an angle of θ=45 degrees relative to the propagation axis of theincident surface wave in an isotropic substrate. Due to the anisotropy,the angle may differ. The groove period (G) 105 can be expressed as:G=λ_(in)cos θ, wherein λ_(in) represents the incident SAW wavelength andθ is the angle of the groove with respect to the SAW propagation axes.

While a typical reflective array compressor employs a continuouslyvarying groove period, in one embodiment of the present design, agreater number of fingers or grooves for a particular frequency may beused to provide sufficient sensing capability. The fingers can begrouped in sections and may even include spacing on one or more sides ofthe grouped fingers to improve isolation. The number of fingers can bebased on a number of parameters according to the design criteria and caninclude the reflectivity of material, the frequency of the section, andthe sensing requirements. For example, the reflectivity of the fingerscan be reduced so that the entire section has sufficient reflectioncharacteristics as opposed to having a highly reflective firstreflective finger. The structure, which resembles a compressor but doesnot follow the associated design rules, is called a reflective arraychannelized (RAC) sensor array. Historically the acronym, RAC, appliesto the compressor and the re-use of the acronym is intentional tounderscore the relationship of the channelized sensor array to thecompressive filter.

Referring to the embodiment of FIG. 1 b, the reflector arrays arearranged to be more discrete and channelized for the sensing pixels byhaving sections of the reflector regions for an incident frequency bandto provide sufficient sensing. An input broadband source 112 such as aninterdigitated transducer (IDT) provides a broadband source that can beas broad as permissible or practical for the given application. Thebroadband source signal 112 is incident upon a plurality of angled inputreflector gratings 114, 124 such that the applicable frequency signalsfor that particular grating is reflected. In this embodiment there is anarray of reflectors starting with a fine pitch high frequency firstinput angled reflector grating 114 such that some of the incidentsignals are reflected. The reflected signals are then incident upon afirst sensing material 116 and then incident upon a complimentary finepitch high frequency first output angled reflector grating 118. Therecan be any number of additional segments 122 of reflector gratings withor without sensing films. There is an N^(th) angled input reflectorgrating 124 that reflects its respective source signal to an N^(th)sensing film 126, wherein the resulting signal is then reflected from acorresponding output reflective grating 128 to the output transducer120. The output broadband transducer 120 receives the various signalsfrom the complimentary reflector arrays. The sensing films can besimilar to one another or entirely different composition, shape or sizeand intended for different sensing.

In operation, according to one embodiment an interdigitated transducer(IDT) launches a broadband signal that is incident on the inputreflector array. There are numerous types of such transducers such assingle phase, multi phase, unidirectional, and bidirectional. And, thereare many different structures that can provide the desired functionalityfor generation of the acoustic energy, including adhered wedgetransducers. The signal is beam-split into a continuous spectrum ofsignals at oblique angles impinging upon a sensing material and is thenincident, typically broadside, onto a complementary reflector array. Thespectrum is recombined into a broadband signal and is directed towards abroadband output transducer.

There is no specific requirement that a broadband signal be employed andit is equally within the scope of the invention to interrogate thespecific desired sensing pixel using a narrowband or single-frequencyexcitation. In such a method, the frequency or band would be alteredsequentially to interrogate a number of desired elements. In practiceone element may offer excellent sensitivity to a number of differentmeasurands but have poor selectivity. The pixel would be interrogatedcontinuously as a trigger and, upon detection of an unspecified threator exposure, the system would then interrogate the more selective butless sensitive pixels to determine the exact composition of theexposure.

FIG. 1 c illustrates another embodiment of the sensing system showingthe grouping of fingers or reflector regions such that the incidentfrequency band on the sensing material is across a section of thesensing material and provides sufficient sensing.

In more particular detail, the broadband source 130 provides thebroadband signal to an array of oblique angled input reflector segments132, 134, 136, 138, 140. It should be understood that there can be anynumber of input reflector segments. Each of the input reflector segmentscomprises a number of reflectors or fingers wherein the number/shape ofthe fingers can be processed for particular design criteria. Therespective frequency for the designated grating section 132, 134, 136,138, 140 is reflected onto respective sensing films 142, 144, 146, 148,150. Once again, there can be any number of respective sensing materialsfor corresponding reflector sections. As noted, the sensing areas can beof different shapes and sizes depending upon the desired sensing.

The respective frequency segments are then incident upon the outputreflectors 152, 154, 156, 158, 160 wherein the output signals arecombined and coupled to the output transducer 170.

According to one embodiment, the input reflector segments 132, 134, 136,138, 140 are not abrupt with respect to sequential segments but ratherhave matching mediums 184 to serve as a transition between the frequencysegments. In one embodiment the reflector array has contiguous segmentswherein the intermediary matching medium 184 would allow for a smoothtransition allowing reflection without undue scattering. The matchingmedium 184 is only shown on one of the reflector array segments forconvenience but may be implemented on all the gratings for both inputand output.

According to one embodiment, certain areas along the reflector array canhave a blocking medium 180 so that there is an attenuated or no signalat that particular frequency and there is essentially a dead zone infrequency response. The blocking medium 180 can be, for example, acoating or stripe of silicone rubber or other polymer applied proximate,such as between channels, to cancel some frequency response and provideimproved isolation. The blocking medium is typically a material thatattenuates or absorbs acoustic waves. The use of a number of blockingmediums 180 can be used in one embodiment to establish a bit codingsequence for that sensor or to implement guard bands. This would allow acoding such that a sequence of blocked frequencies would identify aparticular sensor system.

In a further embodiment, these channels can be coded with a fixedphase-shift code by using a phase shift medium 182. The means andmathematics of matched filter coding, both discrete (digital) andcontinuous (analog) is well known to those in the art. For example, oneillustration of a Barker code SAW device is explained by B. A. Auld,Acoustic Fields and Waves in Solids, Vol. II, New York: Wiley, 1973. Anynumber of channels can have a phase shift medium 182 to uniquelyidentify the particular sensor from among other sensors.

Each channel could have a nominal phase code region that was unique anda sensor region that provided a small perturbation that wassimultaneously measurable but not destructive to the reliableidentification of the code.

By placing distributed pits or phase shifts in the lateral coupling pathit is possible to encode a desired bit sequence into a sensor, makingeach sensor uniquely identified.

As detailed herein, one embodiment of a reflective array sensor is witha continuous sensor region. While it is possible to employ multiplepixels of sensor film, depending upon the usage, the smooth distributionof spectral content may incur significant cross-talk between sensorchannels. The use of wider sensing regions and guard bands with respectto certain channels can be used to increase sensitivity. There are alsoapplications where a single sensor interaction is to be studied as afunction of frequency or of a parameter that varies with position, suchthat the sensing film is the same across a number of frequency segments.One clearly useful example is to establish a temperature gradient alongthe sensor and explore the thermal desorption and adsorption kinetics ofa gas-polymer or an antibody-antigen or a DNA-DNA interaction.

Such a sensor is readily applicable to the frequency-selectivemeasurements of local strain, temperature gradients, or chemicalgradients. One such embodiment would allow a real time gaschromatography system. By inserting the device into a liquid andmeasuring the cut-off frequency of damping, a level sensor is obtained.With proper materials selection to allow fluid phase operation, localviscosity and therefore phase boundaries can be identified. Ofparticular interest is an evaluation of the cross diffusion of antibodyand antigen or of single stranded DNA from opposite ends of the film.Liquid chromatography would be accomplished by observing the migrationin real time as local changes in properties. Selective detection ofreactions between the counter-diffusing biochemicals would result in aslowing or stoppage of diffusion due to size and charge changes.

Referring to FIG. 2, an embodiment is shown for the input half of areflective array sensor having discrete sensor sections (pixels). Thereflective array sensor employs a broadband source 205. Broadband isconstrued to cover multiple sub-bands and not to suggest any specificminimum bandwidth. The broadband source 205, which in this example is aninterdigital transducer (IDT), provides an acoustic beam thatsubstantially defines a distribution path from which the sub-bands areextracted and distributed to the appropriate sensing film pixels 235,250. The broadband source 205 may be unidirectional in order to obtainhigher efficiency, or it may be bidirectional for manufacturing anddesign criteria. In particular, single phase unidirectional transducershave gain-bandwidth limitations that are well known.

Narrowband signals f₁, f₃ are diverted from the distribution path into aspecific frequency-addressable transverse sensing path using obliquereflectors 230, 245. The oblique angled reflector 230, 245 and sensingfilm 235, 250 can be ‘matched’ so that they are optimized for aparticular sensing application. In other words, the angled reflectorscan allow for a certain frequency bandwidth that is optimal for acorresponding sensing film. In one embodiment the gratings 240, 255 canbe used to establish reference signals for the respective angledreflectors 230, 245. The 1^(st) grating 240 reflects some signal f₂which can be used to establish a reference pair with the f₁ signal thatimpinges upon the 1^(st) sensing film. While there may be a differencein frequency, the reference signal f₂ can be used to help establish theparameters from the sensing process. Similarly, the 2^(nd) grating 255and the reflected signal f₄ can establish a reference pair with the2^(nd) reflector 245 to aid in discriminating the sensing filmproperties of the f₃ signal that impinges upon the sensing film 250.

There are some advantages to having one or more sub-bands assigned asreference channels in order to remove the effects of the distributionchannel on the sensor response. Thus, according to one embodiment, oneor more of the narrowband frequencies are not used as environmentallyresponsive sensing measurements.

The signal band from the broadband source 205 is incident upon the1^(st) reflector and reflects some frequency component f₁ to the 1^(st)sensing film 235. Some portion of the signal passes the 1^(st) reflector230 and has some portion of the signal that may be absorbed and/orallowed to continue on to the 2^(nd) reflector 245 after passing throughthe optional 1^(st) in-line grating 240. The acoustic waves f₃ reflectedby the 2^(nd) grating 245 impinge upon the sensing film 250. Once again,the incident signal that passes the 2^(nd) reflector 245 may be incidentupon an optional 2^(nd) in-line reflector 255 and can proceed to othersections, an absorber or an output transducer. As used herein, in-linegrating is essentially a grating that reflects at least some of thesignal back into the path from it came.

According to one embodiment the 1^(st) reflector 230 reflects a firstfrequency and the 1^(st) in-line grating 240 reflects a slightlydifferent frequency f₂ in the RAC structure. Such optional in-linereflectors can return the reflected signal f₂ to the bi-directionaltransducer 205 for one-port embodiments of the sensor. Even though thereflected signals f₂, f₄ from the in-line gratings 240, 255 are at adifferent frequency than the sensing frequencies f₁, f₄, they aretypically close enough to serve as a calibration signal for the sensormeasurements. Thus, in this embodiment, the sensor is self-calibratingas the reflected signals from the in-line gratings 240, 255 provide asignal that is unaffected by the sensing films 235, 250. Alternately byomitting the in-line gratings and employing an in-line output referencetransducer, it is possible to obtain a calibration signal reference fora transmission two-port structure.

By way of illustration of the self-calibrating feature, if the sensordevice employs 10 frequency bits, implemented as five sensor bits andfive reference bits, and each sensor path, f_(2n−1), has an associatedreference path at f_(2n), the response would include sensor measurement(f1), reference measurement (f2), sensor measurement (f3), referencemeasurement (f4), sensor measurement (f5), reference measurement (f6),sensor measurement (f7), reference measurement (f8), sensor measurement(f9) and reference measurement (f10). Frequencies f2, f4, f6, f8 and f10would be reflected back to the input transducer if in-line reflectorswere used or transmitted to the reference output transducer otherwise.

It is noted that there are interferometer systems in which f_(2n−1) andf_(2n) would desirably be equal and there is no contrary indication. Inthis case the oblique reflector should divert only a portion of theenergy so that the transmitted portion is reflected by the associatedin-line reflector. Such an arrangement should be considered whereveroblique and in-line reflector pairs are considered, however the moregeneral case of differing channelization is also to be considered.

There can be an end reflector (not shown) after the sensing films 235,250 wherein the narrowband signals may be reflected directly backthrough the sensing films and oblique reflectors to the bi-directionalsource transducer 205. Alternatively, the signals that go through thesensing film can be reflected to an output transducer (not shown) andsubsequently processed. Alternatively, there can be an in-line outputtransducer (not shown) as a reference source or an optional absorber.

In the case of a bidirectional transducer forward and reversedistribution paths are established. If only one path is sampled, it istypically desired that the other path be absorbed to prevent unwantedreflections, although this is not a requirement.

There are certain manufacturing advantages to extracting the highestfrequency bands closest to the source 205 since lower frequency signalsexperience less degradation in high frequency gratings than vice versa.Thus according to one embodiment, the highest frequency sensing isperformed closest to the source 205 and lower frequency sensing at theoutput. In the case of relatively narrow bandwidths (a few percent orsmaller) there will be little difference. In another embodiment thehighest and lowest frequencies are extracted first and the midbandfrequencies last, allowing the cumulative losses in the distributionpath to be balanced by the higher efficiency at the source transducer'scenter frequency.

Referring to FIG. 3, an illustration of a reflected array response fortwo reflectors and residual signal with three lobes of signal gettingthrough the reflectors is depicted. The following illustrates the sinc²response of a simple IDT source (dashed line—305) with a 100 period IDThaving first signal extraction at 433.25 MHz (solid line—310) and 434.25MHz (dash dot line—320). The remainder 2^(nd) transmitter signal (dashdouble-dot—330) continues to the output transducer or is dissipated inan acoustic absorber. Alternately this signal could be reflected back tothe input as a reference signal.

In this embodiment the signal is in the ISM band. By way ofillustration, in a sensor system designed for the 434 MHz IndustrialScientific Medical (ISM) band, “broad” indicates only about a 0.5%fractional bandwidth, whereas in the 915 MHz ISM band, this indicatesapproximately 3% bandwidth. In other cases, broad could be construed tomean any bandwidth up to the practical limits of the broadband source.

Referring to FIG. 4, another embodiment with a reference transducer isdepicted. Sensors employing transmission measurements will typicallyrequire an input source and at least one output mechanism such as anoutput transducer. As noted, a certain frequency band from the broadbandsource 405 is reflected by each of the reflector segments 410, 430 to acorresponding sensing film 415, 435 wherein the signal is influenced bythe sensing film. The signals in the various transverse sensor paths arethen redirected by additional oblique angled reflectors 420, 440 into alongitudinal accumulation path which is measured by the sensor outputtransducer 425.

One variation includes a reference output transducer 450 located at theterminus of the longitudinal distribution path so that it is notaffected by sensing film. Transmission sensors generally offer higherresolution measurements but are not well suited to passive wirelesssensor systems. In this example, the output transducer 425 receives thesum of the reflected signals and reference transducer 450 would be usedto help detect the residual signal.

According to one embodiment, a guard band 460 is included where anabsorber material is disposed between/proximate adjacent sensing filmsto provide better isolation between the sensing pixels or bits. Theguard band 460 can be deployed around each of the sensing films or onlyabout those sensing measurements that are more difficult to obtain.

Different line widths are within the scope of the invention for the1^(st) reflector segment path 405-410-415-420-425 as compared to the2^(nd) reflector segment path 405-430-435-440-425, as might be requiredif the input and output paths are along different crystalline axes. Thepitch of the reflectors in one pair (e.g. 410 and 420) will typically bedifferent than the other (e.g. 430 and 440). However, for example, in asystem having a total bandwidth of 10% and comprising ten channels eachhaving 1% bandwidths, the variation in pitch from one array to the nextwill merely be 1% and will be difficult to discern visually.

There can be one or more absorbers 455 located in the sensing system,such as the absorbers 455 shown herein to improve signal processing byabsorbing unwanted reflections. Polymers such as photoresist, siliconesand the like are typical materials suitable as absorbers.

The reflective array channelized sensor structure for transmissionmeasurements is well suited to swept frequency measurements (eithercontinuously or discretely stepped), in which each specific frequency isacoustically steered either to the reference output transducer orthrough one of the sensing film regions to the sensor output in asequential manner as the frequency is varied. When the effects oftemperature and other parameters do not significantly move the variousfrequency bands, single frequency per sensor region addressing issuitable. The advantage of this approach over discrete sensor arrays isthat the bands are self-defining relative to one another, allowing aswept measurement to “locate” each band regardless of frequency drift ofthe entire array. Such two-port measurements and structures are oneembodiment for wired applications in which it is simple to connectmultiple transducers (input, reference and output) to multiple points ofinstrumentation. Such wired transmission measurements will place thefewest constraints on device design.

Referring to FIG. 5, a further embodiment includes a sensing systemwherein there is a distinct output transducer on an opposing side to thebroadband source. In this dog-leg path embodiment, the path lengths areapproximately the same mechanical length so that the signals aresynchronous in time, offering little or no “compression” of the originalreflective array compressor filter. The high frequency signals traversethe low frequency signals and in certain embodiments this results inpoor performance, however there are certain implementations that maywarrant such a configuration.

In more particular detail, a broadband source 505 generates thebroadband signal to the array of input oblique angled reflector segments510, 530 and the respective signals are reflected to sensing materials515, 535. The output signals from the sensing films 515, 535 are thenreflected by the output oblique angled reflector segments 520, 540,however in this embodiment the output reflector segments 520, 540 havethe same angular orientation so that the signals are reflected to theoutput transducer 525 which is located opposing the broadband source505. In this manner, if the reflectors are arranged such that thesignals from the broadband source 505 travel from high frequency to lowfrequency, the high frequency signals travel through low frequencyreflectors before arriving at the output transducer 525. In verybroadband systems this is undesirable; however there may be otherpractical reasons with moderate bandwidth. Alternatively, the highfrequency signals can be processed last by reconfiguring the reflectorsso that the low frequency paths are nearest the broadband source.

An application of the invention is as a passive wireless sensor.According to one embodiment, a single transducer serves as both theinput and the output. Referring to FIG. 6, a single common transducer605 provides the broadband source signal to the input oblique angledreflector segments 610, 630 and optional in-line gratings 650, 655. Thecommon transducer 605 also serves as the output transducer.

Reference signals are obtained from in-line reflectors 650, 655 for therespective frequencies wherein the in-line reflected frequencies f₂, f₄can be used as reference signals to discern the sensed properties.Sensor signals f₁, f₃ that are influenced by the sensing film 615, 635are reflected back into the transverse sensor path by end reflectors620, 640 then re-coupled into the longitudinal distribution path by theoblique angled reflectors 610, 630 and coupled to the common transducer605.

As previously indicated, a phase shift material 660 can be deposited inparticular channels to shift the phase characteristics of the channelpath which can be used for coding the sensor unit. In more particulardetail, the phase shift material can be patterned Aluminum or gratingwith different line to space ratio to change the speed of the waveform.Blocking mediums (not shown) can also be included to isolate thechannels.

There is some time delay as the signals are returned to the commontransducer 605. If a band-limited impulse (sine burst) is used tosimultaneously excite all of the channels, the resulting signal willconsist of echoes of each channel, delayed and attenuated by the effectsof the selected sensor films. Signal processing of the reflected spectracan be used to extract the delay time or phase shift and the attenuationin each sensor path, which in turn can be correlated to a sensormeasurands in that region. Resolution and sensor-specific encoding canbe accomplished using matched filter methods. More narrowly band-limitedsine bursts can address individual pixels in the same manner.

The end reflectors 620 and 640 can be continuous, narrowband reflectorsor could incorporate specific bit-sequence codes by having apredetermined reflector sequence introduced to each reflector. As noted,each end reflector 620, 640 is designed for the incident frequency fromthe angled reflector 610, 630. Alternately entire bits can be used forcoding using a phase shift medium or coating 660 or phase plates on oneor more end reflectors to establish a code for the sensor. In the lattercase, interrogation with a complementary bit-sequence would allow aspecific sensor to be identified and also allow processing gain of theotherwise low level spread spectrum signal. As discussed previously, itis possible to use simultaneous encoding and sensing within certaindesign constraints.

Referring to FIG. 7, a different angular orientation is depicted showingthe component orientation. In more particular detail, the broadbandsource 705 provides the acoustic energy to the input reflectors 710, 730wherein the incident signal is directed at a different angle thatreflects the properties of the symmetry of the crystal, such as 60degrees. The signal impinges upon the sensing films 715, 735 and then tothe output reflectors 720, 740 before being extracted by the outputtransducer 725.

In certain embodiments, it is desirable to have a high Q resonance inwireless passive sensors. In particular, sensors which are interrogatedwith a broadband tone burst and in which the resonant frequency isemployed as the sensing parameter require a passive “ring time”sufficiently long to obtain good frequency resolution. Referring to FIG.8, a further embodiment includes a common chirped or discrete array ofoblique angled reflectors 810. The common input/output transducer 805provides the broadband signal that is reflected by the angled reflector810 into the discrete frequency channels. In this embodiment, thecontinuously-varying (chirped) distributed reflector 810 of thereflective array compressor can be employed, ensuring no frequencymismatch between the incident energy and the narrow bandwidthresonators.

In one example, the invention is not limited to the delay line echo modeof operation. By placing semi-reflective gratings in-line grating 815,830 into the transverse sensing path, an array of one port acousticresonators is created. Each resonator is at a unique frequency withinthe band of the source transducer 805. The acoustic energy from thesource transducer 805 is weakly coupled into all of the resonators towhich there is spectral content diverted by the chirped oblique angledreflector 810 thereby charging the resonators with acoustic energy attheir resonant frequency. The end reflectors 825, 840 are typicallycomplete reflectors and reflect back all the incident signals. Theenergy then slowly leaks from the semi-reflective in-line grating 820,830 through the chirped oblique angled reflector 810 and back into thecommon transducer 805.

The constraints on the sensing films, the spacing between reflectors,and other design parameters are routine to one skilled in the art andcan be predicted using coupling of modes or other well known theoreticalframeworks. Generally the sensing regions 820, 835 must either cover thereflectors or be sufficiently short to minimize the numbers of resonantmodes per sensor path in the implementation of FIG. 8. End reflectors825, 840 should have maximum reflection coefficient while the reflectioncoefficients of semi-reflective gratings 815, 830 must be low enough toallow signal insertion and extraction but high enough to provideadequate ring time of the resonator.

There is also no specific width requirement for the lateral sensorpaths, however the widths should be sufficiently large to preventdiffraction and waveguide mode issues. Also, since the reflectors 810are typically at 45° angles, the spread in location versus frequency issuch that the desired frequency is likely to be distributed over aregion as wide as the width of the source transducer or wider.

By way of further illustration, since per-element reflectioncoefficients much in excess of 1% lead to scattering into bulk waves, a100-element (50 wavelength) lower limit on the coupling reflectors isused in one embodiment. Since narrow transitions between frequencychannels are desired, even more elements are desirable and practicalwidths for the lateral sensor beams are on the order of 100-200wavelengths. Reciprocity demands that for complete reflection back intothe main beam the width of the longitudinal distribution path, andtherefore of the transducers, must be comparable. At 300 MHz thewavelengths are on the order of 10 microns (1-2 mm beams) and at 915 MHzthe wavelengths are on the order of 3 microns (0.3-0.6 mm beams). A20-channel sensor with 2 mm beams might be 6 mm wide and 50 mm long,allowing standard wafer scale manufacturing to obtain a moderate numberof 20-measurand sensors per 75 mm or 100 mm piezoelectric wafer at thelowest practical frequencies.

The continuous sensor is also suitable for passive wireless applicationsby an extension of the “end reflector” of the discrete sensor. Referringto the one port device of FIG. 9, the common transducer 905 launchesenergy into the longitudinal distribution beam where it is distributedby angled reflector 910 having continuously variable pitch into sensingregion 915. The signals then impinge upon the slanted finger endreflector 920 and the signals are reflected back through the sensingregion 915. In this embodiment, the slanted finger reflector (SFR) 920is designed to have the same local period as the angled reflector 910 inany lateral cross-section through the oblique angled reflector, sensorfilm, and slanted reflector. Note that a plurality of sensing films maybe employed and that this example is not meant to be limited to a singlemeasurand.

Such a sensor allows a wireless interrogation via an antenna (not shown)into the transducer 905 to interact with the entire sensor region 915using the local pitch of reflectors 910 and 920 to define a frequencyselective region of interaction along the length of the device.

An advantage of this sensor embodiment is that all of the varioussensing films are incorporated into a single device, sharingstatistically correlated manufacturing tolerances and operating in a“self aligned” process. By placing each individual sensing film into afrequency-addressable path of the same sensor, it is feasible to placemore individual sensor functions within a given bandwidth or volume.

The art related to encoding an individual device are varied.Fundamentally either an impulse is applied and the output is convolvedwith a matched filter or a matching interrogation signal is transmittedwhile the impulse resulting from convolution of the interrogation codeand the sensor's encoding is sought for. In practice it is likely thatneither the transmission nor the retransmission would employ an impulsefor many power handling and noise immunity reasons. Instead, athree-part convolution in which a spread spectrum signal is transmitted,embedding a portion of the matching code's Fourier transform with atransmission spectrum, X(F), is used. The retransmitted signal would bethe product of this spectrum and the sensor's filter function, S(F). Thereceiver would then further filter with a receiver filter function,R(F), such that X(F)*S(F)*R(F)=exp(−jωτ), being a time delayed impulse.There are numerous other methods of encoding. In practice, the “filter”functions X(F) and R(F) are implemented in software; the “impulse” hasfinite width (is band limited); and the sensing mechanism causes S(F) todeviate from approximating (X(F)*R(F))⁻¹. Nonetheless practical codingschemes follow some analog of this process and the problem ofintroducing several programmable phase shifts into the channels of S(F)as a bit code and then determining X(F) and R(F) to decode the bits ismathematically mature and computationally efficient.

The existing signal processing for reflective array compressor filtersexclusively involves Rayleigh SAW devices. However, some of theapplications for the technology are in biochemical and other fluid phasesensing applications with certain special considerations. There arematerial substrates typified by 36° rotated Y-cut lithium tantalate and41° rotated Y-cut lithium niobate that offer extremely highpiezoelectric coupling to a nearly pure shear horizontal SAW (SH-SAW)along the X-axis of propagation. The propagation at right angles to X ischaracterized by a pure shear wave with no piezoelectricity and a pureRayleigh wave. One embodiment is a shear horizontal RAC (SH-RAC) sensorusing such orientations to address these applications, using theteachings herein along with the careful selection of orientation andwave guiding structures. SH-RAC may be implemented using the approachherein, for example, with crystals of symmetry 3 m. Bluestein-Gulayevwave orientations, such as on lithium tetraborate offer anotherembodiment. An SH-RAC substrate is defined herein as a material andassociated surface plane supporting piezoelectrically coupled SH-SAWpropagation in at least one direction and a pure-SH wave, with orwithout piezoelectric coupling, in another direction. In other words, asubstrate having at least one axis of propagation along which atransducer launches a shear horizontal wave and another axis into whichit can be reflected with no mode conversion. An SH-RAC shall be definedas a RAC device, as described herein, manufactured on an SH-RACsubstrate.

There exist alternate methods of distributing a broadband input signalover a spatially diverse array of sensing locations or pixels. One suchmethod uses the composite transducer of a slanted array compressorfilter to create the slanted array channelized (SAC) sensor array. Acomposite transducer consists of several laterally translatedsubsections, each optimized to a different frequency band. In thecompressor, the subsections have continuously overlapping frequencybands; however in the sensor implementation there is a desire forspatial orthogonality of the various acoustic beams. This is readilyaccomplished through “poor” compressor design, as was pursued in the RACembodiment. For example each transducer may have a weighting function(apodized or withdrawal weighted) to introduce a desired pass-band shapewith low spurious responses and steep skirts. The individual segmentscan be better spaced in frequency than would be desired in a compressor.Finally, the individual segments can carry fixed encoding to make themnot only spatially and spectrally orthogonal but also code orthogonal.

According to one embodiment, a slanted array compressor's composite IDTis used to introduce and distribute a broadband electrical energy signalsource into multiple, narrow-band, channelized frequency bands by astaggered, segmented, source transducer such that the channels arerouted to parallel, frequency-addressable paths of narrow-band, acousticenergy signals. These acoustic energy signals are then transmittedthrough the sensing material and thereafter either reflected back byappropriate reflectors or retrieved and reassembled into a broadbandaltered electrical energy signal by a staggered, segmented, receivertransducer. The use of the slanted array compressors provides one way tocompensate for distortion and second order effects such as thosedescribed for the reflective array compressor, which requires that thesignal traverse unrelated reflector sections (e.g. f₃ traveling throughf₁ and f₂) and introduces certain negative effects on the signalprocessing. To provide an alternative, the slanted array compressor canbe implemented wherein the slant array compressor slants the transmitterand receiver transducers at some angle wherein the source transducer andthe receiver transducer are coupled in parallel.

Referring to FIG. 10, a slanted array channelized (RAC) sensor array1000 is depicted with a pair of slanted source transducers 1005, 1010having a plurality of frequency dependent segments 1030, 1040. Thetransducers are arranged and slanted in such a manner that the segments1030, 1040 are equidistant and have essentially the same delay componentbut at different frequencies f₁, f₂, f₃-f_(N), wherein every segmentrepresents a different frequency band. The acoustic energy from thesource transducer 1005 impinges upon sensing areas 1020 and thecorresponding sensed response is received in the receiver transducer1010.

An advantage of this SAC sensor array embodiment is that there is onlyone propagation direction and it is inherently chanellized using adiscrete implementation. This may be particularly well-suited forwireless implementations as it may have less loss.

The use of SH-SAW and other shear horizontal substrates is desirable forliquid phase applications of the SAC and is within the scope of thisinvention.

The use of interferometric techniques is accomplished by individuallytransmitting f_(n) and f_(n+1) into adjacent channels using separate IDTsegments but receiving them in common using a single output segment.

According to one embodiment, certain sensing films 1020 are missing inorder to provide reference signals that are close to the sensedfrequency thereby facilitating self calibration.

Coding of the sensor can be accomplished in several ways depending uponthe number of sensors deployed. For example, since the channels aredistinguished by different frequencies, a wideband device canaccommodate a large number of different frequencies and the coding canbe done in the frequency domain. For example, certain frequency bandscan be nulled to identify a particular sensor such as by using a coatingor blocking medium. Coding can also be done by the arrangement ofin-line reflectors to provide reference signals at certain frequenciesin order to distinguish devices. Several other techniques fordistinguishing devices using frequencies are also known to those skilledin the art.

In another embodiment, a metal film can be used, similar to the phaseplate noted herein, wherein each segment can be subjected to a phasecoding in order to distinguish the sensor. MEMS devices can beincorporated into the structure to provide a switching means to modifythe coding and therefore identify a sensor. By coding with such a bitstream, the sensor is addressable and sends back correlation pulses.

In more specific detail of one embodiment, a metal layer film that mayor may not be continuous can have chunks at each frequency for someseparation of the spatially diverse areas. Blocks can be established atF1, F2 . . . and these would not chirp, making it easier to interpolatebetween readings. With respect to the coding, any arbitrary number ofpixels or bits can be implemented depending upon the operations. If usedas an RFID tag, it can be used so that phase shifting creates + and −bit sequences in frequency so that if interrogated instead of a signalpulse but with the right opposite code, it will come back with ‘1’ or‘0’ for an n bit code. Various coding schemes can be implemented such asBarker code and Hamming code. In sensor applications, it is now possibleto measure uniform loading using factors such as temperature, strain,and torque with an RFID tag that can be interrogated by a code among aplurality of RFID tags.

The frequency band associated with a particular segment 1030, 1040 istypically a function of the manufacturing process resolution andgenerally can be a very wideband overall frequency with many differentsegments.

In this particular embodiment, the linear phase for each segment is thesame and the transmission time/delay is the same. While depicted thateach segment 1030, 1040 is about the same size, a further embodimentincludes having the segments of different sizes. For example, certainsensing applications may require a more sensitive reading having greaterresolution or simply be more difficult to sense, wherein the device canhave an elongated segment.

In the various radar implementations, guard bands were not typicallyused, however in one embodiment of the sensing application describedherein, a guard band is employed on at least one of the segments inorder to improve the isolation characteristics. For example, aparticular measurand may be more difficult and require a more accurate.

Referring to FIG. 11, another type of slanted compressor device isdepicted. Once again, the compressor provides channelized frequencycomponents however in this non-linear embodiment the time delay betweenthe segments is different and the phase between segments is different.As noted, the distance L₁ is shorter than the distance L_(N) and thusthere is a time delay differential and a dispersive response.

According to one embodiment, a phase plate 1150 can be disposed betweenthe source transducer and the receiver transducer. The phase platetypically is a thin metal film that is designed to provide for phasecorrection and used to obtain a quadratic phase using a phase plate byincluding some metal for each channel.

The geometry of the phase plate 1150 selectively perturbs the velocityof the acoustic waves and in applications such as pulse compressionradar, the signal waveforms are adjusted to ensure a quadraticdifferential phase response. In one embodiment the phase plate 1150 isused to provide a phase shift coding to change the phase code for eachfrequency band by use of a grating structure having bumps and indents1155. Such usage of the phase plate 1150 allows the sensor to be codedto distinguish the sensor from among other sensors. For example, thephase plate 1150 can establish a code pattern for each segment such asπ/2, 3 π/2, π/2, 3 π/2 . . . In certain applications, such as trying tomeasure the temperature of a concrete structure having multipletemperature sensors, the individually addressable sensor can be excitedwith a signal that is pre-coded for that sensor. The reflective arrayembodiments can also include a phase plate.

In another embodiment, multiple bands can be incorporated into the phaseplate, especially with a strong coupling material, implementing gratingsthat can provide open/short circuits to adjust the phasecharacteristics. The open/short circuits can be switched in anotherembodiment to dynamically adjust the phase characteristics and make auniversal phase plate. The phase plate provides an easy mechanism toprovide coding and implement orthogonal frequency coding. The phasemodulation for each channel can therefore establish a signature for anindividual sensor.

Another variable factor for the present invention relates to thetransducer geometry. There are various knows types of transducer designssuch as split-electrode transducers, dummy fingers, stepped-fingers,curved finger electrodes and slanted finger electrodes. One suchgeometry is the slanted finger interdigital transducer (SFIT) whereinthe fingers of the IDT are designed for an application to have slantedfingers.

Another embodiment exists in which the composite transducer is replacedwith an interdigital transducer having a local pitch and centerfrequency that varies along the lateral dimension of the transducer.

Referring to FIG. 12, a slanted finger interdigital transducer (SFIT)sensor configuration is shown wherein the acoustic wave signals from theSFIT vary along the length of the device. Similar to the reflectivearray example of FIG. 1 a, this variation does not have discrete andseparate channels, but rather has a continuum of geographicallydispersed spectral content.

The input source IDT 1205 has a plurality of slanted fingers 1220 thatconvert a broadband electrical energy signal into a continuum ofacoustic energy signals with a frequency that varies along the width ofthe transducer 1205 (length of the individual slanted electrodes). Theacoustic energy is input to at least one sensor path 1250 by frequencyaddressable excitation due to the spatial localization of coherenttransducer operation along the lateral dimension (top to bottom in thefigure) and then input to a receiver transducer 1210. The receivertransducer 1210 also has slanted fingers 1230 that are designed to havecoherent detection at frequencies and locations that match the frequencygenerated by the input transducer 1205 at the corresponding laterallocation. Put simply, the acoustic wavelength is coherent from left toright and vice versa along any horizontal cross-section through thetransducers and intervening sensing area and varies continuously alongany vertical cross-section.

It should be understood, that the RAC and SAC sensor arrayimplementations are just a few of the possible combinations that arewithin the scope of the invention. There are various sensingrequirements that may require a mix of gratings, absorbers, sensingmaterials and transducers in some combination not explicitly shown inthe figures. Such a configuration is intended to be within the scope ofthe invention and only illustrative examples are noted herein. The useof a SAC or SFIT transducer to extent the bandwidth of the transducersin a RAC is also explicitly considered.

According to one embodiment, dispersive acoustic wave devices offer ameans for frequency-selective addressing of spatially distinct sensingor phase encoding elements, and this class of devices offers significantadvantages over traditional acoustic wave devices in sensingapplications. It should be noted that the deviation from linear phase isnot the salient property and even non-dispersive devices made from thesedispersive elements offer the advantages disclosed. In one embodiment,the use of frequency channelization to address a desired sensor pixel inan array.

Referring to FIG. 13, one embodiment of the system disposed upon asubstrate is depicted. The coupling to an external antenna 1305 with anoptional matching network 1310 shows how signals can be received andtransmitted according to one embodiment. In this embodiment, the commontransducer 1320, reflector section 1325, 1335, in-line gratings 1330,1340, sensing films 1345, 1355 and end reflectors 1350, 1360 are alldisposed upon a substrate 1315.

The individual reflectors that make up the reflector section 1325, 1340can be grooves disposed on or about a surface of the substrate 1315 withthe geometry and groove period defining how the incident acoustic beamis processed. In this embodiment the high frequency signals areextracted first by the fine pitch or smaller groove period of the firstreflector section 1325. The reflected spectrum exiting the reflectorsection 1325, 1340 impinge upon the sensing materials 1345, 1355 withthe corresponding frequency and may be influenced by the sensing filmthereby providing altered reflected signals. The altered reflectedsignals are then reflected back to the reflector sections and back tothe common transducer 1320. Likewise, the in-line gratings 1330, 1340reflect back some portion of the impinging signal that can be used as areference in certain embodiments.

Certain applications in biochemical and other fluid phase sensingapplications have special considerations. There are material substratestypified by 36° rotated Y-cut lithium tantalate and 41° rotated Y-cutlithium niobate that offer extremely high piezoelectric coupling to anearly pure shear horizontal SAW (SH-SAW) along the X-axis ofpropagation. SH-RAC may be implemented using the approach herein, forexample, with crystals of symmetry 3 m. Bluestein-Gulayev waveorientations, such as on lithium tetraborate offer another embodiment.SH-RAC substrate is a substrate material and associated surface planesupporting piezoelectrically coupled SH-SAW having at least one axis ofpropagation along which a transducer launches a shear horizontal waveand another axis into which it can be reflected with no mode conversion.

In a wireless sensor application, in which encoding would be helpful, itis also desirable to have a single electrical port. As noted herein, onemeans of accomplishing this is to employ reflective arrays to fold theacoustic energy back onto the common transducer. Another is to simplyconnect the input and output transducers. A third method is to replacethe complementary reflective array with an array of traditionalnarrowband in-line reflectors, each returning the incident energy backto the beam-splitting reflector and subsequently back to the inputtransducer. All of these approaches are illustrated in some fashion inthe preceding examples, singularly or in combination, or are otherwiseunderstandable from the figures.

A two-port device can be implemented using a second, complementarycomposite transducer. A one-port device can be implemented using anarray of narrowband transducers. In this embodiment the compositetransducer provides the functionality of the input transducer and thebeam-splitting grating. All of the coding and sensing mechanisms of theRAC can be applied to the SAC and vice versa.

Another embodiment for reflective array sensors relates tonontraditional substrates. There are applications in which the scale ofmeasurement is unsuitable to microlithography, wherein it is possible toexcite surface waves on a stainless steel rod or other macroscopicstructures using the well-known wedge transducer. A simplehalf-wavelength resonator is affixed onto a wedge and the wedge isaffixed onto the plate or rod such that the bulk waves in the wedgecouple to surface waves on the substrate. Etched or machined grooves inthe plate serve the same function as the metal strip reflectors typicalof microlithographically manufactured SAW devices.

A number of mechanisms can be used to generate acoustic wave energy ontothe substrate. From signal processing viewpoint, wedge transducers arenot advantageous, however for gross sensing applications a wedgetransducer can be satisfactorily employed according to the inventionsdetailed herein. One implementation is a transmission sensor consistingof a rod or bar of corrosion-resistant elastic material such asstainless steel or met-glass, supporting an input and output transducerof the wedge transducer type, said input transducer converting bulkacoustic energy into a beam of surface guided acoustic energy, saidenergy propagating through one or more oblique angled reflectors. Thetransversely reflected signals at varying frequencies are thenredirected by one or more complementary oblique angled reflectors tosaid output wedge transducer. The apparatus is sealed at the wedges,preventing damping of the surface waves by the sealant but isolating theacoustic wave sensing structure from the electrical connections to thewedge transducers. Upon immersion into a liquid to a controlled depth,those frequency components traveling into the liquid loaded area willsense the properties of the liquid or will be completely damped byliquid loading. The apparatus thus serves as a dipstick or depthprofiling sensor. By virtue of the exclusion of piezoelectric elementsor electrical contacts in the sensing environment it is intrinsicallysafe and explosion-proof. Such a sensor would experience significantapplicability to chemical processing and fuel tank monitoring.

The use of a shear transducer and/or a bi-layered substrate can furtherprovide a sensor with macroscopic scale suitable for liquid phasemeasurements. Such a sensor could be produced to be several meters inlength and could serve to monitor phase boundaries and liquid levels inlarge vessels as well as to measure viscosity as a function of depth.

In another embodiment, the arbitrary angle of the input obliquereflector array is selected to be 180°. That is, the chirped reflectoris designed to reflect different frequencies directly back to the sourcetransducer at different distances from the source transducer. In onesuch embodiment, an accurate level sensor can be obtained by employing,for example, a metal column such as stainless steel. The column wouldhave etched grooves to create the (now in-line) chirped reflectivestructure. A piezoelectric material can be bonded or disposed on thesurface of the column such that it launches extensional bulk waves ofchannelized acoustic energy and converts said energy into a SAW orsimilar wave in the column. A waveguide or slot may be used to focus theenergy and lower the problems of reflections and lateral coupling. Byexamining the reflected signal response, the level is determined by thecut off frequency.

In a further embodiment, the acoustic energy is provided by lasers thatare phase matched with the system. By way of illustration, a tiltablelaser optic system to change the angle over time can be a broadbandsource.

According to one embodiment, frequency addressablemicroelectromechanical systems (MEMS) devices are incorporated toprovide the bit coding sequence wherein the MEMS open and close toprovide a path for the signal reflections. The MEMS can gate theacoustic energy and block incident energy.

Sensors of this type using a polymer or other substance, as might beused in a chromatography column, disposed between the complementaryreflectors may be used to measure the time-evolution of diffusion of aliquid or vapor sample through the polymer, creating a real-timechromatograph.

In one embodiment, the sensors employ a polymer or other film, toselectively detect or bind a certain analyte which is disposed betweenthe complementary reflectors and is used to evaluate thefrequency-dependent effect of the film-analyte reaction on the acousticwave propagation. Such a device probes the frequency dependentviscoelastic interactions and provides data redundancy for enhancedselectivity.

According to one embodiment, sensors use a sequence of discrete regionsof polymers or other films, each interacting with a predetermined bandof the spectrum through spatial diversity of the signal, may be used toimplement a multi-analyte sensor with frequency-addressableinterrogation.

By way of example, sensors may be encoded by placing regions of a filmto introduce a sequence of 0° and 180° or other transmission phasesbetween the complementary reflectors to create a coded impulse response.Such devices can be used as code-addressable physical sensors in whichthe entire coded response varies with a parameter such as pressure,temperature, strain or the like. Combining chemically insensitive 0° and180° (or other encoding phase value) encoding regions with(bio)chemically selective sensing regions it is possible to encode a(bio)chemical multi-analyte sensor.

While the prior art in signal processing employs a smoothly varyingreflector pitch to obtain a desired phase-frequency response for pulseexpansion or compression, the sensor application would benefit fromdiscrete beams of essentially monochromatic acoustic waves. Provided thetwo reflectors are complementary, it is possible to implement a largenumber of such monochromatic beams, the most significant limitationbeing crystal size.

According to another embodiment, the techniques herein apply to anydispersive device technology for spatial diversity. One of theembodiments is a device that tracks inherently due to structure of asingle device that gets around needing to have separate sensors for eachthe different analytes.

Some applications include tire pressure monitoring systems whereinmultiple wireless sensors are presently required such that the sensor isidentifiable and addressable.

According to one embodiment the system operates by employing certainspread spectrum features. A spread spectrum signal and power is spreadout over time and frequency and run through a complimentary device suchthat a reconstituted signal is a sharp pulse. This is used in radarapplications instead of sending out a large power narrow pulse.

With respect to certain sensing application, the dispersive aspects arenot important and it is the spatial routing of the signal employed toobtain dispersion that is sought. The system allows addressing a localeon the intervening area by what frequency the sensor is interrogated.Multiple sensing films and each with certain frequency characteristicscan be used wherein one can see what is happening at a certain film byinterrogating at that frequency.

By way of example, instead of four different sensors with four differentcarefully crafted frequencies that split the ISM band with correspondingguard bands, the present invention can use one sensor that can beswept/switched between the sensors and provide the measured information.

Certain reactions between polymers and the acoustic wave devicesunderneath them are frequency dependent. So in certain implementations,such as biosensor arrays, one may obtain an improvement in separationbetween what is changing in the polymer compared to other effects withinthe device by employing a swept frequency measurement with onebiochemical or polymer along the entire sensing region.

In signal processing reflective array compressor filter applications,the fine grating between the fingers is typically never perfecttherefore the chirp response is imperfect. Thus in one embodiment, asufficiently wide track is used between the reflective arrays so a metalfilm can be patterned so the local length of the metal is the phasecorrection of the device, correcting to quadratic phase so that thedelay time is linear with time. While this is not needed in sensorembodiments, if there is a metal film that is laser trimmed to obtaincoding, one can obtain phase shift keying for RFID coding.

It should be understood that the present invention is applicable tovarious devices such as SAW, BAW and silicon-based MEMS type resonatordevices.

In yet a further embodiment, the entire RAC or SAC structure isimplemented onto a cantilever structure such as those described in U.S.patent application Ser. No. 11/753,047, which is incorporated byreference herein for all purposes In this embodiment the RAC or SAC canoccupy some or the entire cantilever surface and form an active acousticregion.

The foregoing description of the embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthis disclosure. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto.

1. A sensing apparatus, comprising; an acoustic energy source providingan input acoustic energy; a first reflector array having a plurality ofindividual reflectors reflecting a portion of said input acoustic energyand wherein said first reflector array provides a plurality of reflectedsignals with differing frequencies; at least one sensing regionproximate said first reflector array, wherein at least some of saidreflected signal impinges upon said sensing region providing an alteredreflected signal; and a transducer receiving at least some of saidaltered reflected signal.
 2. The apparatus of claim 1, wherein theindividual reflectors are selected from the group consisting ofcontinuously varying individual reflectors and sections of similarreflectors.
 3. The apparatus of claim 1, further comprising a secondreflector array proximate said sensing region with said sensing regiondisposed between said first reflector array and said second reflectorarray, wherein said second reflector array reflects at least some ofsaid altered reflected signal.
 4. The apparatus of claim 1 wherein saidtransducer receiving said altered reflected signal is selected from thegroup consisting of: common transducer which also provides said inputenergy source and separate output transducer.
 5. The apparatus accordingto claim 1, further comprising at least one of the group consisting ofin-line gratings reflecting at least some of said input acoustic energy,in-line gratings reflecting at least some of said reflected signal,in-line gratings reflecting at least some of said altered reflectedsignal, and end reflectors reflecting at least some of said alteredreflected signal.
 6. The apparatus according to claim 1, furthercomprising an absorber material proximate at least a portion of aperiphery of said apparatus.
 7. The apparatus according to claim 1,further comprising a reference transducer receiving at least some ofsaid input acoustic energy.
 8. The apparatus according to claim 1,further comprising at least one microelectromechanical system (MEMS)device proximate said sensing region.
 9. The apparatus according toclaim 1, further comprising a medium on at least a portion of saidsensing region, said medium selected from at least one of the groupconsisting of: blocking medium and phase shift medium.
 10. The apparatusaccording to claim 1, further comprising a matching medium on at least aportion of said first reflector array.
 11. The apparatus according toclaim 1, wherein said apparatus is disposed upon a shear horizontal (SH)RAC (SH-RAC) substrate.
 12. The apparatus according to claim 1, whereinsaid sensing region is selected from at least one of the groupconsisting of: polymer films, metal films, metal oxide films, enzymes,antibodies, DNA, and thin membranes.
 13. A sensor system for detecting atarget analyte, comprising: providing an initial acoustic energy signal;reflecting portions of said acoustic energy signal by a reflector arrayto a sensing region, producing at least one altered acoustic energysignal, wherein said reflector array has a plurality of individualreflectors, and wherein said reflector array reflects more than onefrequency; receiving said altered acoustic energy signal; convertingsaid altered acoustic energy signal into an altered energy signal; andmeasuring a change between said initial energy signal and said alteredenergy signal for said detecting of said target analyte.
 14. The systemaccording to claim 13, further comprising reflecting said alteredacoustic energy signal by an additional reflector array to an outputtransducer.
 15. The system according to claim 13, wherein said acousticenergy source is a common transducer and further comprising reflecting aportion of said altered acoustic energy signal back to said commontransducer.
 16. The system according to claim 13, wherein said sensingregion is selected from at least one of the group consisting of: polymerfilms, metal films, and metal oxide films, enzymes, antibodies, DNA, andthin membranes.
 17. The system according to claim 13, further comprisingcoding by at least one of the group consisting of: disposing a blockingmedium on portions of said sensing region and disposing a phase shiftmedium on portions of said sensing region.
 18. A slanted array sensor,comprising: a slanted source transducer converting a broadbandelectrical energy signal into a plurality of channels of narrow-bandacoustic energy signals; at least one sensing region proximate saidslanted source transducer, wherein at least one of said narrow-bandacoustic energy signals impinges upon said sensing region and producesat least one altered acoustic energy signal; and a slanted receivertransducer proximate said sensing region, wherein said slanted receivertransducer receives at least one of said altered acoustic energysignals.
 19. The sensor according to claim 18, wherein said slantedsource transducer is a slanted finger interdigital transducer and saidslanted receiver transducer is a slanted finger interdigital transducer.20. The sensor according to claim 18, further comprising a phase platedisposed between said slanted source transducer and said slanted receivetransducer.